1. Field of the Invention
The present invention relates to a current-drive circuit and apparatus for display panel, and particularly to a current-drive circuit and apparatus allowing a display panel to incorporate display elements thereon so that uniformity in light-emission intensity is improved.
2. Description of the Related Art
In recent years, as semiconductor elements become smaller and smaller in response to progress in micro-processing techniques, LSI (Large Scale Integrated Circuit) incorporating such semiconductor elements becomes larger and larger. For example, in a display device such as a liquid crystal display device, an output circuit, provided in a drive circuit, for driving data lines receives 8-bit digital data for display of one pixel and produces voltages for display of 256 gray scale 2-dimensional images so that the voltages are applied to and then drive liquid crystal, in order to achieve a liquid crystal display panel capable of displaying 16,770,000 colors.
That is, when analog images are converted to digital data, an 8-bit or 16-bit signal is used to relate a gray scale to a particular intensity level. To reproduce monochrome images, 1-bit information, i.e., 2 gray scale representation, in which “0” and “1” represent black and white respectively, is used as a minimum number of gray-scale levels.
On the other hand, as is known in the art, to reproduce color images, three primary colors, red (R), green (G) and blue (B), are blended. For example, when red (R), green (G) and blue (B) are represented at 256gray scale levels, totally, 16,770,000 colors can be displayed according to the calculation: 256×256×256=16,770,000.
A current-drive device employed in a drive circuit for such display panel is disclosed in Japanese Patent Application No. 13 (2001)-42827. The conventional current-drive device disclosed in the above-described publication is configured to include a plurality of current-drive Integrated Circuits (hereinafter, referred to as ICs) connected in series as shown in FIG. 1. Referring to FIG. 1, a plurality of current-drive ICs 1 through 4, each employing a current mirror circuit as a constant current source, and a reference current source 5 are inserted between a high voltage supply and a low voltage supply, and current mirror circuits incorporated within each of the plurality of current-drive ICs are connected in cascade to allow current passing through the plurality of current-drive ICs to become approximately equal to one another.
When current mirror circuits within the above-described current-drive IC consist of a MOS transistor, variations in the threshold voltage VT of MOS transistor increases variations in currents passing through the current-drive IC chips in proportion to the number of current-drive ICs.
Another current-drive device employed in a drive circuit for display panel is disclosed in Japanese Patent Application No. 14 (2002)-244618 and shown in FIG. 2. Referring to FIG. 2, the current-drive device comprises a current supply unit 22 and a sink-current adjustment unit 23. The current supply unit 22 includes reference current sources I1, I2, . . . . , In for sourcing different levels of currents, and a plurality of switches SW1, SW2, . . . , SWn configured to receive currents from the reference current sources I1, I2, . . . , In and switch between ON and OFF states in response to control signals D1, D2, . . . , Dn, thereby appropriately combining currents from the reference current sources I1, I2, . . . , In to output current of a particular level. In this case, the plurality of switches SW1, SW2, . . . , SWn have one ends connected respectively to the reference current sources I1, I2, . . . , In and the other ends connected together. The sink-current adjustment unit 23 receives the reference current of a particular level as a result of outputs from the switches SW1, SW2, . . . , SWn and then adjusts the level of sink current, and further, outputs the sink current of a particular level to one of data lines connected to individual pixels.
The above-described example represents a general current-drive circuit and when each of primary colors is to be represented, for example, at n-bit gray scale levels, the current-drive circuit supplies current of a particular level by combining binary-weighted constant currents I1 to In.
However, a current-drive circuit for supply of binary-weighted constant currents cannot guarantee monotone increase in output current when output current to be supplied to a display panel is monotonically increased because adjacent constant currents are different from each other by a factor of 2. Therefore, the current-drive circuit is neither able to increase or decrease current with high resolution nor to supply current for representing a particular color at the greater number of gray scale levels. Furthermore, the above-described current-drive circuit cannot apply gamma correction with high accuracy to output current corresponding to a digital signal.
Still another conventional drive device employed in a drive circuit for display panel is disclosed in Japanese Patent Application No. 13 (2001)-350439. The image display device disclosed in the above publication applies gamma (γ=2.0) correction to drive current corresponding to a digital signal by adjusting both the level and pulse width of drive current. However, since the drive current comes to have small pulse width at the representation of lower number of gray scale levels, the drive current that is able to drive a light-emitting element at a particular brightness level cannot potentially be supplied.
As described above, in case of the conventional current-drive apparatus for display panel disclosed in Japanese Patent Application No. 13 (2001)-42827, the current-drive apparatus is constructed such that a plurality of current-drive ICs IC1 to IC4 are connected in cascade and current mirror circuits are connected in cascade within each of the plurality of current-drive ICs IC1 to IC4, and nearly equal current is generated to flow within each of the plurality of current-drive ICs IC1 to IC4. However, when each of the current mirror circuits is constituted by MOS transistors, variations in threshold voltage of MOS transistor unfavorably increase the degree of variations between the current-drive ICs in proportion to the number of the current-drive ICs.
Furthermore, in case of the current-drive apparatus disclosed in Japanese Patent Application No. 14 (2002)-244618, combining any of binary-weighted constant currents I1 to In makes it difficult to supply current for representing a particular color at the greater number of gray scale levels because monotone increase in output current as an aggregation of the binary-weighted constant currents from the current-drive circuit is deteriorated. Furthermore, the current-drive circuit cannot apply gamma correction with high accuracy to output current corresponding to a digital signal.
Additionally, in case of the current-drive apparatus disclosed in Japanese Patent Application No. 13 (2001)-350439, the image display device applies gamma correction to drive current corresponding to a digital signal by adjusting both the level and pulse width of drive current. However, when the drive current becomes very small in magnitude, the response speed of the drive current in a MOS transistor circuit is potentially lowered.